1. Field of the Invention
The instant invention is directed to methods of and apparatus for manipulating electromagnetic phenomenon. More particularly, the instant invention is directed to methods of and apparatus for manipulating electromagnetic phenomenon to convey information utilizing substantial portions of visible and invisible spectra to manipulate information, whether that information is in the form of pictorial displays, such as holographic displays; in the form of numerical information, such as digital or analog numerical information; or in the form of communication signals, wherein the signals are selected from any portion of the electromagnetic spectrum.
2. Prior Art
One of a number of fields to which the instant invention is pertinent is the field of holography. In order to create a holographic image, it is necessary to superimpose two coherent light beams which are incident on the same photographic plate or other suitable recording device. One beam is known as the "object beam" and the other beam as the "reference beam". In "off-axis holography", the beams are separated by an angle .theta., which is typically 45.degree.. The term "off axis" is utilized because the angle .theta. between the beams results in the axes of the beams not being coaxial. The general equation of holography is exceedingly complex and does not lend itself to any solution other than a numerical solution. The complexity of the equation is readily apparent when one considers two coherent waves S.sub.r (the reference wave) and S.sub.b (the object wave) with the waves being incident on the same photographic plate or other recording device, wherein: EQU S.sub.b =A.sub.b e.sup.-i.theta. EQU S.sub.r =A.sub.r e.sup.-i.alpha.
Since the intensity at any point (x,y) in recording medium is given by the equation: EQU I=I(x;y) x I*(x;y)
therefore, EQU I=A.sup.2.sub.b +A.sup.2.sub.r +A.sub.b A.sub.r exp{.theta.(x;y)+.alpha.(x;y)}+A.sub.b A.sub.r exp[-i(.theta.(x;y)-.alpha.(x;y)}]
There is no limitation on the content of S.sub.b and S.sub.r, but, in accordance with the practices of the prior art, one is a "plain wave", while the other is modulated with object information.
It can be readily seen that if .alpha.=.theta., waves S.sub.b and S.sub.r are coaxial, resulting in "in-line" holography. When .alpha.=.theta., the equation becomes greatly simplified. However, the prior art approaches have not produced viable holograms because prior art in-line devices produce double images, zero order aberrations, or both. The zero-order problem is a long-standing problem and is set forth in U.S. Pat. No. 3,944,322, incorporated hereby reference and assigned to Polaroid Corporation. An in-line system was demonstrated by the originator of holography, Dennis Gabor, who in 1971 received a Nobel Prize for his contribution. Gabor's device produced primitive holograms which, due to a lack of a coherent light source, were incapable of storing three-dimensional images. Off-axis configurations in which .alpha. and .theta. are different solve the double-image problem, but in many situations they are unsatisfactory because they are cumbersome and require a large air space to disperse the zero order light. In essence, off-axis configurations for off-axis holograms require considerably more space for operation than on-axis holograms. Exemplary of commercial off-axis holographic systems are the bar code readers used for checkout scanners in stores and disclosed in U.S. Pat. No. 4,415,224, incorporated herein by reference.
By substituting on-axis systems where off-axis systems are now used in the field of holography, considerable advantages will result in that if the art of holography is drastically simplified by using on-axis systems, holographic devices can be made much smaller and used for applications which in the past, if not unsuitable for holographic technology, have used that technology ineffectually.
A specific example of commercially available off-axis holographic technology is the use of a hologram head-up display to display information on the windshield of an automobile. For example, Nissan Corporation offers as an option a display of speedometer information in the left-hand corner of the windshield. This display is accomplished by an off-axis hologram and is not particularly satisfactory because the hologram is wiped out under certain ambient light conditions, is invisible when viewed through polaroid glasses, and is not conveniently displayed in the driver's line of sight. A system for displaying such a hologram is disclosed in U.S. Pat. No. 4,902,082, incorporated herein by reference.
While holography is of interest with respect to displaying images and information, it is also of interest in communications wherein information is transmitted via fiber optic cables. (See, for example, Monomode Fiber-Optic Design, pages 454, 455, incorporated herein by reference.) In order to optically transmit information, it is necessary to multiplex and demultiplex the information as it is fed into and read from a fiber optic bundle. (See, for example, J. Hecht, Understanding Fiber Optics, Computers and Local Area Networks, page 371, 1987, incorporated herein by reference.) In accordance with prior art techniques, off-axis holography is utilized. Typical of such techniques is the holographic fiber optic multiplexer, demultiplexer system developed by Ingwall, Kolessinski, and Fielding of the Polaroid Corporation, wherein a photopolymer film, especially developed for infrared light, is used to make a holographic element which simultaneously collects, separates, and focuses light from information-carrying optical fibers. In that the holographic optical element creates separate channels within each fiber, the amount of information carried by a fiber can be dramatically increased. This approach is used to expand telephone service and to provide increased capacity for computer linkage. However, since the holographic signal multiplexer/demultiplexer utilizes an off-axis configuration, difficulties arise in alignment when the holographic element is coupled to an optical fiber. Light must be incident on the core of the fiber and injected into the fiber at an angle relative to the fiber axis that is less than the acceptance angle. Only light within the acceptance cone, formed by rotating a ray at the acceptance angle about the axis of the core, will propagate in the fiber. When one utilizes an object beam which is axially displaced from the reference beam, proper alignment is compromised, resulting in attenuation and dispersion of the signal. This decreases the number of bits of information that can be transmitted over a given fiber in a specified period and thus reduces cable capacity.
Further, with respect to holographic fiber optic multiplexers and demultiplexers, it is difficult to transmit information in both directions when one must deal with optical elements which utilize an object beam which is angularly displaced from a reference beam. This effectively halves the amount of information which can be transmitted.
Multiplexing and demultiplexing of information is not only of significance with respect to fiber optic communications, but is also of interest with respect generally to the processing of information. However, when it is necessary to axially displace the object beam which carries the information from its reference beam, which renders the information intelligible, it is difficult to process information utilizing holographic principles in that coupling holographic elements together becomes exceedingly complex. This complexity is readily apparent when one considers the basic holographic equation (see above), wherein the angles .alpha. and .theta. must be dealt with. In off-axis holography, the angles .alpha. and .theta. do not cancel one another, whereas in on-axis holography such cancellation occurs because the object and reference beams are coaxial.
While holography is generally considered to be a phenomenon primarily of interest with respect to the entire electromagnetic spectrum for the display of images utilizing complex light beams or manipulation of optical information utilizing laser beams, many of the same principles which are applicable to visible light are also applicable to other portions of the electromagnetic spectrum such as infrared and ultraviolet radiation, radiowaves, and X-rays. There is a need for a device which extends the optical principles utilized in holography to other manipulations of electromagnetic radiation.
In the past, conical elements have been utilized to manipulate light. For example, devices known as "waxicons" and "axicons" have been explored for optical extraction in lasers (D. Fink, "Polarization Effects of Axicons", Applied Optics; Vol. 18, No. 5, pg. 581, 582; Mar. 1, 1979, and J. W. Ogland, "Mirror System for Uniform Beam Transformation in High Power Annular Lasers", Applied Optics, Vol. 17, No. 18, pg. 2917, Sep. 15, 1978. However, no suggestion has been made that these devices can be utilized to minimize zero-order problems.
The production of 3-D television systems relying on holography has been explored for years (R. E. Zammit et al., "Compatible Color 3-D TV system: Proposed Design", Applied Optics, Vol. 18, No. 5, pg. 584, 585, Mar. 1, 1979). Use of on-axis holography would greatly simplify making 3-D, holographic television a possibility.